The invention relates to a method of controlling the charge air mass flow of an internal combustion engine, which is supercharged by an exhaust gas turbocharger having adjustable vanes. A control unit compares the actual control value with a desired control value and provides control signals to a vane adjusting mechanism for adapting the actual value to desired value.
As is well known, an exhaust gas turbocharger includes a gas turbine driven by the exhaust gas flow of an internal combustion engine and a compressor, which is driven by the turbine and which compresses a fresh air flow for the internal combustion engine depending on its speed. The turbine and the compressor are disposed on a single shaft so that they are rigidly interconnected. As a result, the charge air pressure generated by the compressor and the exhaust gas pressure upstream of the turbine are intertwined because of the torque equilibrium at the charger shaft. Because of the charge air pressure effective at the exhaust side of the compressor, the exhaust gas stream upstream of the turbine is backed up. The pressure head of the exhaust gas flow is converted by the turbocharger to charge air pressure in accordance with a particular pressure transmission ratio which is effective between the turbine and the compressor and which is determined by the respective flow cross-sections. When the charge air pressure is increased, also the charge air mass flow, that is, the charge air flow through the internal combustion engine increases so that its power output is increased.
It is well known that the inlet flow cross-section of the turbine can be changed by an adjustable turbine geometry, for example by adjustable inlet vanes. In this way, the energy of the pressure head, which is to be transmitted by the exhaust gas turbocharger to the charge air flow can be changed. The turbine geometry can assume any position between a fully open position with maximum inflow cross-section and a fully closed position with minimum inflow cross-section and the vane position is infinitely variable between the fully open and fully closed positions. In a stationary operating state of the internal combustion engine, by controlling the turbine geometry the inlet flow cross-section of the turbine is controlled so as to provide the desired charge air flow to the engine. With increasing power, the inflow cross-section of the turbine is reduced and the resulting backup air pressure head of the exhaust gas increases the compressor output. In this way, the charge air mass flow is adapted to the respective operating conditions. During instationary operation of the internal combustion engine, for example during a load increase step, the turbine geometry is changed so as to reduce the inlet flow cross-section to increase the charge air pressure. A load increase step is the incremental load increase between two stationary operating conditions.
The adjustable turbine geometry is the control structure of a control unit for controlling the charge air mass flow. The control unit performs a comparison between an actual and a desired value of the control structure with a control value provided as a guide value. Depending on the actual/desired value comparison of the control value, the control unit generates control signals for a controller for adjusting the turbine geometry in order to adapt the actual value to the control value.
Such a method is already known from DE 195 31 871 C1 for controlling the charge air pressure, wherein a control difference of the charge air pressure, that is the difference between the given desired value and the determined actual value is supplied to the control unit. If the determined actual value of the charge air pressure deviates from the given desired value, the control unit changes the control setting of the adjustable geometry of the turbocharger inlet vanes to cause an adaptation of the actual value to the desired value. The desired value for the charge air pressure to be provided is selected from a particular performance graph and a respective value is taken therefrom depending on the measurement data of the control deviation as a function of the engine speed and the fuel injection amount. When the inlet flow cross-section to the turbine is increased in order to speed up the turbocharger and to increase the charge air pressure, the back pressure of the exhaust gas at the outlet of the internal combustion engine is of course also increased. This increases the exhaust work that is the gas change work of the piston, which decreases the efficiency of the internal combustion engine. It is therefore necessary that the exhaust gas back pressure is monitored and an excess pressure is counteracted so that the engine torque increases in an optimal manner.
The known method proposed therefore to supply to the control unit an additional input value, which is determined as a function of the exhaust gas pressure. This additional value represents the difference between an operating point-dependent maximally admissible differential pressure and the actual differential pressure, the differential pressure being calculated as the difference between the exhaust gas pressure and the charge air pressure. This difference can assume values, which are negative, about zero or positive. If, for example, as control value a differential is determined, which is much smaller than zero, the exhaust gas pressure is too high for an optimal torque buildup. Then the control algorithm of the charge air pressure is modified in such a way that the vane opening of the exhaust gas turbocharger is corrected toward greater opening. The control unit can access for this process a differential pressure performance graph from which it can take the maximally admissible differential pressure for a possible correction of the vane position of the turbine geometry.
In order to provide a suitable control algorithm and to be able to handle, in addition to the control deviation, the difference between the charge air pressure and the exhaust gas pressure, the known method needs an additional fuzzy-control unit.
The adjustment of the control process to various operating conditions of the internal combustion engine over the whole engine performance range and particularly during stationary operation requires numerous and complex tests for the establishment of a nominal charge air pressure performance graph for determining the control deviation of the charge air pressure. Particularly for load control (engine speed=constant) and for acceleration procedures (load and engine speed not constant), the known control method on the basis of the charge air pressure, the charge air performance graph and the controller adjustment must be established in a time consuming manner on an engine test bed by a highly experienced application expert.
It is the object of the present invention to provide a method of controlling the charge air mass flow which, with little application efforts, provides for an optimal operation behavior of the internal combustion engine under any operating condition.